1. Field of the Invention
The present invention relates to a capacitance detecting circuit used for a capacitor sensor to detect capacitance with the amount of charges, and to a fingerprint sensor using the same.
2. Description of the Related Art
Personal authentication technologies have attracted considerable attention in recent years to ensure various securities, and a technology of authenticating fingerprints using a fingerprint sensor among them is gradually put into practice in terms of the convenience of personal authentication of a cellular phone and the like.
As one example of the fingerprint sensor, an area-type fingerprint sensor is shown in FIG. 12, in which column lines and row lines (FIG. 12A) are formed on two films at a predetermined gap therebetween, and in which, after a necessary insulating film is formed on the upper surface of the film, the films are bonded to each other with an air layer (gap) or elastic insulating material interposed therebetween (FIG. 12B).
This area-type fingerprint sensor functions as a pressure sensitive sensor in which the shape of the film is changed corresponding to the shape of the fingerprint when a finger is in contact with the sensor, so that the gap between the column lines and the row lines is changed due to the unevenness of the fingerprint, thereby detecting the minute variation of capacitance as electrical signals based on the change of the gap.
As shown in the area-type sensor of FIG. 13, which is an equivalent circuit of FIG. 12, it is necessary to detect the minute change in capacitance in a matrix shape even less than several femtofarads (fF) on a point basis of each of the intersections where a plurality of column lines and a plurality of row lines intersect each other.
Therefore, the capacitance detecting circuit used for the area-type sensor must have high measurement sensitivity since the variation of capacitances at the intersections is very small.
In order to detect the variation of electrostatic capacitance, a well-known technology applicable to such high-sensitivity capacitance detection uses generally a capacitance detecting circuit having the structure of a charge amplifier using a feedback capacitor (Cf) and an operational amplifier, as shown in FIG. 14 (see Japanese Unexamined Patent Application Publication No. 8-145717 (Corresponding U.S. Pat. No. 5,633,594)).
However, according to the capacitance detecting circuit of the related art, the amount of charges of the capacitor to be detected is converted into a voltage level, based on the relationship between a predetermined capacitance and the voltage, using the configuration of the charge amplifier having the operational amplifier, as shown in FIG. 14, in order to detect the variation of capacitance.
In the charge amplifier, feedback control is carried out so as not to cause the variation of the input voltage. Therefore, there is a merit in that the charge amplifier is not affected by parasitic capacitance.
However, in the case of the area-type fingerprint sensor, the capacitance detecting circuit may require several channels.
In this case, in order to take the configuration of the charge amplifier shown in FIG. 14, for example, the operational amplifiers and feedback capacitors must be integrated in the several hundred units to correspond to the number of sensors.
In addition, an output voltage Vout is expressed as follows:Vout=(Csen/Cf)×Vin,
where Csen is capacitance to be detected (the amount of the variation of capacitance in a capacitor formed at each intersection of the sensor), Cf is the capacitance of the feedback capacitor, and Vin is a driving voltage for driving the column lines (which is output from the column line selecting circuit of FIG. 13).
Accordingly, by means of a ratio of Csen to Cf, the gain of an output voltage to an input voltage is set.
As described above, in order to configure the charge amplifier in the capacitance detecting circuit, each of the operational amplifier and the feedback capacitor must be integrated into an integrated circuit so as to correspond to the number of detection outputs of the sensor.
However, when the variation in capacitance of the sensor is detected to drive the column lines, the output voltage exceeds the power supply voltage of the operational amplifier to be saturated when Cf is small in the case of the variation of capacitance Csen of the capacitor at the intersection.
However, when Cf increases to correspond to the capacitance Csen of the intersection, an area where the feedback capacitor is formed increases, and a circuit area for the charge amplifier increases in response to the value of Cf, which causes a chip including the capacitance detecting circuit to be large-scaled, resulting in an increase in manufacturing costs.
Further, when a general-purpose capacitance detecting circuit is employed, gain adjustment is required in response to the area-type fingerprint sensor for the target of use, and the feedback capacitor having several capacitance values is prepared so as to perform adjustment on the gain, which are combined with each other to correspond to the respective targets.
As a result, an area for the feedback capacitor also increases due to this combination, which in turn causes the chip area to be increased, resulting in an increase in manufacturing costs.